Ok, so maybe I'm over thinking this. So I should just
mount switches in the centers, make sure I'm jogged close
to a known side on start up, and just home on them.

Yup, that's how it is set up on my Bridgeport. The X trip
bar is in the center, and the Z is near full up. The Y is
at one end, but the Y travel is pretty short, anyway.

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So now I'll have to work out switches that are repeatable
to 5 microns. I'd think focused optical if you could keep
it clean would be the way to go, I don't know.

Do you have encoders on this machine, or is it step-driven?
No sense in making the home switches any more accurate than
an encoder count or distance of a step pulse. If your
encoders have an index pulse, then you can have it refine
the home position to the encoder index pulse after finding
the home switch. That is how my machine works.